Our U.S. patent application Ser. No. 523,891 now U.S. Pat. No. 3,989,105, describes a multiple-tube heat exchanger which is particularly suitable for use as a steam generator and in which the primary heat-carrying fluid is formed by the cooling fluid (pressurized water or liquid sodium) of a nuclear reactor. The exchanger described in the parent patent comprises, in combination, an outer heavy structure in the form of an outer cylindrical shell fixed at its upper end to an annular intake head and an annular outlet head for one of the fluids, of which the inner bases are perforated, and an inner lightweight structure subsequently mounted in the heavy structure and formed by an annular nest of tubes inserted and welded at their ends into the perforations in said bases and arranged in the form of concentric layers forming a large central passage which is accessible for installation of the tubes, beginning with the peripheral layer adjacent the outer shell, the concentric layers of tubes being held in several transverse planes by discs formed by concentric rows of rings arranged between these layers and joined together by radial stays, a wide central shaft subsequently being fixed for internally defining the annular exchange chamber.
In one embodiment described in earlier Specification, the tubes of the exchanger are all identical and comprise arcuate bends situated successively on either side of the longitudinal axis of the tubes, which provides them with a quasi-sinusoidal form, the retaining planes, in the form of concentric rings joined together by connecting elements allowing through the fluid circulating along and around the tubes, being situated level with the crests of the sinusoids or level with the inflexion points and being joined by welding to the outer cyclindrical shell and to the central shaft.
It will be appreciated that it is possible, by using tubes shaped in this way, to compensate by elastic flexural deformation the differential expansion which can occur between the tubes and the outer shell in view of the fairly significant differences in temperature, which may amount to as much as 200.degree. C, between the tubes and the shell, which would obviously not be the case if the tubes were straight. As indicated in the earlier Specification, the result obtained in this way is a considerable reduction in the stressing which the tubes undergo. However, this method of fixing the tubes has to satisfy two opposing requirements to enable maximum benefit to be derived from the flexibility of the tubes. On the one hand, the manner in which the tubes are fixed should leave them with maximum freedom of expansion, whilst on the other hand the tubes have to be fixed fairly rigidly in order to prevent them from vibrating. In the arrangement described in the earlier Specification, this result was obtained by varying the distance between the rigid retaining planes of the tubes because the tubes can expand more freely, the longer the curve along which their free expansion occurs, whilst widening the intervals between the fixing points involves the danger of vibration of the tubes. Thus, it is extremely difficult to establish a satisfactory compromise between these two opposing requirements. The present invention enables this result to be obtained by simple and effective means.